This invention relates to a method and apparatus for nondestructive evaluation of structures and more particularly to a method and apparatus for remotely exciting vibrations in a structure to be evaluated and remotely measuring and analyzing the vibrations to obtain data related to physical attributes of the structure.
It is often necessary to evaluate engineering structures non-destructively either after such structures are built as a quality control inspection or after some period of service, particularly where the service is rigorous and the structures are critical to environmental or personnel safety thus requiring reliability assurance. Typical of non-destructive evaluation (NDE) technologies are ultrasonics, eddy current techniques, radiographics and acoustic emission analysis. Each of these and other conventional NDE techniques are applicable to particular types of structures and particular types of evaluations. In a situation where one technique is efficacious others may not be and there are situations where none of the conventional techniques is adequate. An example of such situation is the need to evaluate nondestructively the bond integrity of the thermal protection system of the space shuttle orbiter.
The thermal protection system comprises fiber composite tiles bonded to flexible strain isolation pads which, in turn, are bonded to the outer surface of the orbiter. Repeated missions in hostile environments can cause degradation of the bond integrity through heat and moisture. Due to the unique nature of the fiber composite tiles and flexible strain isolation pads between the protective tiles and the space shuttle orbiter, conventional NDE techniques have proven inadequate. In any case, most of the conventional techniques involve physical contact with the structure being evaluated and evaluate relatively small areas on the structure. To reliably verify the integrity of more than 30,000 tiles on an in-service space shuttle orbiter, an evaluation approach that can make measurements rapidly and remotely is desirable. The present invention is addressed to the problem of remote NDE.
According to the present invention, the structure to be evaluated such as a heat-protective tile bonded to the surface of a space shuttle orbiter, is excited to a resonant vibrational mode by an audible acoustic signal. The vibrational mode is analyzed to obtain information relative to the integrity of the bond. A significant element of this analysis and thus a significant aspect of the novelty of the present invention, is the method and apparatus used for measuring and characterizing the vibration.
Prior art methods of measuring and characterizing vibration include accelerometers, which are glued to the surface of the structure being evaluated. Clearly, the accelerometer does not provide the remote sensing capabilities that are desired.
A prior art, non-contacting vibration measurement technique is image dissector tube technology. This is a well developed technology in which the image of an edge of an object is tracked as it vibrates to generate the displacement signal. An image of the edge is formed on an input screen which generates photoelectrons which can then be steered with deflection plates to maintain a constant position of the edge on an output phosphorous screen. Any deviation in the position of the edge output image can then be sensed to generate an error signal which is fed to deflection plates to bring the edge back to the reference position. The error signal is proportional to the edge displacement. This technology has several shortcomings which preclude its use for tile vibration measurement. Most significant, the vibrations of interest are of such small amplitude that the image dissector tube does not provide a sufficient signal to noise ratio. In addition, measurements of out of plane displacements require the tube to be at an angle with respect to the displacement vector. There is an additional problem of being restricted to measuring at an edge rather having the capability to measure an arbitrary point of the tile.
Another prior art means for measuring vibrations is a fiber optic sensor which measures the displacement of the surface by monitoring the intensity of light which is emitted from one fiber or fiber bundle, reflected off a vibrating surface and received by a second fiber or fiber bundle. Since the vibration of the surface affects the amount of light deflected from transmitting to receiving fibers, a measurement can be made of the surfaces displacement down to submicrometer levels at band widths up to 70 KHz. The major drawbacks of this technology are extremely small standoff distances (typically less than 0.1 mm) to achieve adequate sensitivity and an inability to obtain an adequate sensitivity with a diffuse and somewhat variable surface, such as found on the orbiter tiles.
Another technology for measuring vibrations that has been described in the literature is optical heterodyning. Optical heterodyning makes use of the fact that a laser beam reflected off a vibrating surface will be frequency shifted by the surface movement. By causing the frequency shifted reflected beam to interfere with a reference beam, a beat frequency signal is obtained whose phase or instantaneous frequency shift follows the surface vibration. Suitable demodulation electronics can then be used to extract a signal indicative of the velocity and/or displacement of the surface. The applicant has found that optical heterodyning produces an inadequate signal for measuring vibrations from surfaces that are diffusely reflective such as the protective tiles of the space shuttle orbiter.
It is thus an object of the invention to provide a non-destructive evaluation method and apparatus which includes a non-contacting accusto-optic vibration sensing capability. It is a further object of the present invention to provide a system for remotely measuring vibrations of submicron displacements at frequencies up to 5,000 Hz.